The instant application relates to a heat flow meter, and method, for testing thermal properties of materials including thermal conductivity and/or heat capacity.
Thermal properties, such as thermal conductivity, are important physical properties of solids. Heat flows through a solid that has a temperature gradient across its volume. The thermal conductivity of a specimen can be measured directly by measuring the heat flux resulting from a know temperature gradient across a known thickness.
A one-dimensional form of the Fourier heat flow relation is sometimes used to calculate thermal conductivity under steady-state conditions: k=Q (ΔX/ΔT), where “k” is thermal conductivity, “Q” is a heat flow per a unit surface area (heat flux), and ΔT is a temperature difference over the thickness ΔX.
Prior Art FIG. 1 illustrates a conventional static heat flow meter for measuring the thermal conductivity of a test sample (e.g., piece of insulation such as fiberglass). The test sample or specimen is located between two flat plates, and the plates are maintained at known, but different, temperatures. As heat flows through the test sample from the hot side to the cold side, a heat flux transducer (not shown) measures the amount of heat transferred. Thermocouple(s) or other temperature measuring device(s) measure the temperatures of each of the two plates (i.e., of the so-called hot and cold plates). These values are then plugged into the above-listed equation, so that the thermal conductivity of the test sample or specimen can be calculated based on the measured values. Such measurements are often done in accordance with standard testing methods such ASTM C 518, which is incorporated herein by reference. It is in such a manner that insulation products such as fiberglass batts are assigned so-called “R-values”—based on their steady state or static measured thermal properties per ASTM C 518 (e.g., R11 fiberglass insulation batt, etc.).
Unfortunately, the standard testing device of FIG. 1 discussed above determines thermal properties of the test sample via steady state or static testing, where there is no air flow (i.e., there is zero air movement introduced into the testing equipment during the testing). Thus, measurements from such devices can be deceiving as will be explained below.
When insulation (e.g., fiberglass insulation batt, fiberglass loose-fill, cellulose loose-fill, combination/laminate of fiberglass and foam insulation, or the like) is provided in a vertical wall cavity of a home (e.g., between two-by-four studs as is known in the art), it has been found that air flow (e.g., due to wind or the like in the environment surrounding or adjacent to the home) through the wall can have an affect on insulation properties. Contributions to total building heating or cooling load include the change in enthalpy of air moving through an insulation (e.g., fiberglass) and the heat flux through the insulation due to the imposed thermal gradient. The two effects are not independent since the air movement affects the temperature distribution in the insulation. One may experience an example of air flow in an exterior wall of a home by feeling a cool draft in the winter when one puts his or her hand adjacent an electrical outlet. Such air flows in or through walls can reduce the thermal performance of insulation, since insulation such as fiberglass is not an air barrier as it does not stop air flow.
Heretofore, there has been no efficient way to measure the effect of air flow on insulation products. In particular, there has been no way to quantify how much air flow reduces the thermal performance of certain insulation products. Unfortunately, the conventional heat flow meter shown in FIG. 1 and discussed above does not take air flow into account when measuring thermal properties of the test sample.
In view of the above, it will be apparent to those skilled in the art that there exists a need in the art for a heat flow meter, and method, for measuring thermal properties of a product (e.g., insulation product) in a manner which takes into account dynamic air flow.